Create a new array from an existing array that holds the count for how many times an item exist using javascript/jquery

Question

I have a list of items in an array and i want to create a new array that is an array of objects with the key being the value from the existing array and the value being the number of times that item was in the existing array. Hope that makes since, if not hopefully this example will help. No third party libraries like lodash or underscore please.

Any help would be greatly appreciated, thank you.

var arr = [a,a,a,b,c,c,d,e,f,f,f,f];
new array =[{a:3},{b:1},{c:2},{d:1},{e:1},{f:4}];

My attempt so far:

            countryCodes.sort();
            var result = count(countryCodes);
            console.log(result[0]+": "+result[1]);
            function count(arr) {
                var a = [], b = [], prev;
                arr.sort();
                for ( var i = 0; i < arr.length; i++ ) {
                    if ( arr[i] !== prev ) {
                        a.push(arr[i]);
                        b.push(1);
                    } else {
                        b[b.length-1]++;
                    }
                    prev = arr[i];
                }
                return [a, b];
            }

Yes many times. I feel like their is a simple answer and i have been staring at the code for too long. — Travis Michael Heller
– Travis Michael Heller, Commented May 2, 2016 at 15:09
Ok, please edit the question to add your attempts to solve the problem — Tushar
– Tushar, Commented May 2, 2016 at 15:10
var arr = ['a', 'a', 'a', 'b', 'c', 'c', 'd', 'e', 'f', 'f', 'f', 'f']; var count = {}; arr.forEach(function (e) { count[e] = count[e] ? ++count[e] : 1; }); var res = []; Object.keys(count).forEach(function (key) { res.push({ [key]: count[key] }); }); console.log(res); — Tushar
– Tushar, Commented May 2, 2016 at 15:19

recursive · Accepted Answer · 2016-05-02 15:28:46Z

2

You're close.

var a = "aa",
    b = "bb",
    c = "cc",
    d = "dd",
    e = "ee",
    f = "ff";

var arr = [a,a,a,b,c,c,d,e,f,f,f,f];

function getCount(arr) {
  var result = {};
  
  for (var val of arr) result[val] = result[val] + 1 || 1;
  
  return result;
}

var totals = getCount(arr);
document.write(JSON.stringify(totals));

answered May 2, 2016 at 15:28

recursive

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2 Comments

Travis Michael Heller Over a year ago

it's almost like magic on how you come up with some of these answers.They make complete sense when i read them but coming up with on my own is going to take a lot more practice. Your answer was actually exactly what i was looking for even though i did not ask for it in the correct wording.Thank you.

Redu Over a year ago

Neat..! I liked the shortcut.

t1m0n · Accepted Answer · 2016-05-02 15:18:53Z

1

Open console when press run button.

var arr = ['a', 'a', 'a', 'b', 'c', 'c', 'd', 'e', 'f', 'f', 'f', 'f'];

function getObjectFromArray(a) {
  var result = [],
      alreadyAdded = [];
  a.forEach(function(item) {
    var key = item,
      obj = {};
    if (alreadyAdded.indexOf(item) != -1) return;
    alreadyAdded.push(item)
    obj[key] = getCount(a, item)
    result.push(obj)
  })

  return result;
}

function getCount(a, el) {
  var count = 0;
  a.forEach(function(element) {
    if (element == el) count++
  })

  return count;
}

console.log(getObjectFromArray(arr))

answered May 2, 2016 at 15:18

t1m0n

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1 Comment

Travis Michael Heller Over a year ago

Thank you @t1m0n. I definitely need to practice my arrays and objects, as i get a little lost when trying to figure out these kind of situations. Again, thank you.

Lunny · Accepted Answer · 2016-05-02 15:50:57Z

Here is another working way.

   function count(arr) {
    var prev = arr[0];
    var current;
    var newArr = [];
    var count = 0;
    var obj = {};
    for (var i = 0; i < arr.length; i++) {
        current = arr[i];
        if (i == arr.length - 1) {
            if (prev == current) { 
                count++;
                obj = {};
                obj[prev] = count;
                newArr.push(obj);
            }
            else {
                obj = {};
                obj[prev] = count;
                newArr.push(obj);
                count = 1;
                obj = {};
                obj[current] = count;
                newArr.push(obj);
            }
        }
        else if (prev == current) {
            count++;
        }
        else {
            obj = {};
            obj[prev] = count;
            newArr.push(obj);
            count = 1;
            prev = current;
        }
    }
    return newArr;
}
        console.log(count(['a', 'a', 'a', 'b', 'c', 'c', 'd', 'e', 'f', 'f', 'f', 'f']));

Redu · Accepted Answer · 2016-05-02 16:14:34Z

1

This can be a very simple task with Array.prototype.reduce()

var arr = ["a","a","a","b","c","c","d","e","f","f","f","f"],
    res = arr.reduce((p,c) => { var i = p.findIndex(e => !!e[c]);
                                !!~i ? p[i][c]++ : p.push({[c]:1});
                                return p},[]);

document.write("<pre>" + JSON.stringify(res) + "</pre>");

OK the below one should be faster like o(2n) and it's nothing more longer than two lines of object assignments.

var arr = ["a","a","a","b","c","c","d","e","f","f","f","f"],
    mid = arr.reduce((p,c) => {!!p[c] ? p[c]++ : p[c]=1; return p},{}),
    res = Object.keys(mid).map(e => ({[e]:mid[e]}));

document.write("<pre>" + JSON.stringify(res) + "</pre>");

edited May 2, 2016 at 16:14

answered May 2, 2016 at 15:44

Redu

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3 Comments

recursive Over a year ago

This has runtime in O(n^2), which might matter if you are trying to process a large list.

Travis Michael Heller Over a year ago

This is using ecma6 and i am unable to use babel as a third party library. But good to know and well written, thanks!

Redu Over a year ago

@Travis Michael Heller thank you... the second snipped should be ok with ES5 though...

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